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Production method for sintered bearing member, fluid dynamic pressure bearing device, and spindle motorRelated Patent Categories: Bearings, Rotary Bearing, Fluid BearingProduction method for sintered bearing member, fluid dynamic pressure bearing device, and spindle motor description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20050259898, Production method for sintered bearing member, fluid dynamic pressure bearing device, and spindle motor. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to production methods for sintered bearing members, fluid dynamic pressure bearing devices, and spindle motors, and in particular relates to production techniques for sintered bearings which are used in spindle motors rotating with high precision for information devices. [0003] Bearing devices using sintered bearing members have been used for spindle motors installed in optical disk devices in the field of information devices and for spindle motors used for polygonal mirrors installed in laser printers, and the like. Recently, these bearing devices have been also used for spindle motors installed in magnetic disk devices (hard disk drives). Fluid dynamic pressure bearing devices are used for spindle motors installed in magnetic disk devices as the bearing devices, and are required to have higher levels of performance such as high precision rotation, low noise, shock resistant, and low bearing torque loss even in the case of using sintered bearings. Therefore, several techniques have been researched for sintered bearing members to efficiently generate dynamic pressure. [0004] The following techniques regarding the fluid dynamic pressure bearing devices have been proposed. For example, Japanese Patent Application Laid Open No. 7-216411, in the Abstract, proposes a technique to reduce friction coefficient and improve sliding performance, in which pores of at least a sliding surface of a sliding member is sealed by an impregnant to reduce the frictional resistance to sliding, thereby improving the frictional performance of the sliding member. Japanese Patent Application Laid Open No. 11-62948 proposes a technique to reduce production cost and ensure high precision rotation, in which a bearing surface facing an outer surface of a shaft is formed on an inner surface of a bearing body made from a sintered alloy, and is formed with dynamic pressure grooves inclined toward the axial direction of the bearing body, whereby the shaft is supported in a floating condition by air dynamic pressure generated by relative rotation of the shaft and the bearing body. [0005] Japanese Patent Application Laid Open No. 2002-333023 proposes a technique to produce sintered sliding bearings with few processes, high quality, and stable yield performance, in which a resin is impregnated into pores of a sintered body and hardened therein; the sintered body is inserted into a die and compressed to reduce or close by plastic working the clearance formed between the pore and the resin due to shrinking during hardening of the resin. [0006] Conventional fluid dynamic pressure bearings obtained by the techniques in the above proposals can ensure precise high speed rotation and low noise compared to ball bearings. Furthermore, the bearings can support large loads by suitable design and forming of the dynamic pressure grooves on the bearing surface. The dynamic pressure grooves on the bearing surface are formed in a sintered bearing member by plastic working. The sintered bearing member is produced by compacting a metal powder and sintering a green compact, and is a porous material usually having about 20% porosity, whereby lubricating oil is impregnated into pores, and the bearing member is used in several kinds of motors. The fluid dynamic pressure bearing device used for spindle motors which is required to have precision rotation as mentioned above is formed with dynamic pressure grooves to generate dynamic pressure on a bearing surface, and a member rotating relative thereto is supported by the dynamic pressure. [0007] An ordinary sintered bearing member made of a porous material has a disadvantage that the dynamic pressure generated on a bearing surface leaks through pores, thereby decreasing load supporting capacity. As means for preventing leakage of the dynamic pressure, it has been known to impregnate a resin into pores and seal in a production process of the sintered bearing member. By such a treatment, decrease of the dynamic pressure generated on the bearing surface is avoided and the rotating member can rotate with high precision. [0008] When the dynamic pressure grooves are formed on the bearing surface, for example, in a case of a sintered bearing member having multiple circular arc grooves, such as three circular arc grooves shown in FIG. 3, a pin having protrusions corresponding to the three circular arc grooves is inserted into an inner peripheral surface of a ring-shaped sintered material into which a resin was impregnated, and the pin is extracted therefrom, whereby the shape of the three circular arc grooves is transferred to the inner peripheral surface of the sintered material. For example, in a case of a sintered bearing member having herringbone grooves shown in FIG. 4, a pin having protrusions corresponding to the herringbone grooves is inserted into an inner peripheral surface of a ring-shaped sintered material into which a resin was impregnated, and the outer peripheral surface of the sintered material is compressed to abut the inner peripheral surface to the pin, whereby the shape of the herringbone grooves is transferred to the inner peripheral surface of the sintered material. [0009] When the three circular arc grooves or the herringbone grooves are repeatedly formed by the above-mentioned plastic working in the ring-shaped sintered material into which a resin was impregnated, the resin impregnated into the sintered material may adhere to the surface of the pin which forms the dynamic pressure grooves. If the resin adheres to the surface of the pin which forms the dynamic pressure grooves, the shape of the dynamic pressure grooves is not precisely transferred to the inner peripheral surface of the sintered material. Furthermore, scratches may be formed on the inner peripheral surface of the sintered material when the pin is extracted during the forming of the three circular arc grooves in the sintered material, and the scratches may result in leakage of dynamic pressure. SUMMARY OF THE INVENTION [0010] Therefore, objects of the present invention are to provide a production method for a sintered bearing member, a fluid dynamic pressure bearing device, and a spindle motor in which the shape of the dynamic pressure grooves can be precisely transferred to the inner peripheral surface of the sintered material; formation of scratches can be avoided when the pin is extracted in producing the sintered material having the three circular arc grooves, and leakage of the dynamic pressure can be avoided, thereby being suitable for information devices, such as magnetic disk devices, with high reliability. [0011] The present invention provides a production method for a sintered bearing member, the method comprising: compacting a metal powder into a green compact; sintering the green compact into a sintered compact; impregnating a water soluble resin solvent into the sintered compact; and removing the resin solvent adhering to a surface of the sintered compact, thereby exposing a matrix as sintered on a surface of the sintered compact. Pores existing in portions other than the surface are sealed with the resin. [0012] The present invention further provides a fluid dynamic pressure bearing device having a radial sintered bearing member, the sintered bearing member being produced by steps comprising: compacting a metal powder into a ring-shaped green compact; sintering the green compact into a sintered compact; impregnating a water soluble resin solvent into the sintered compact; and removing the resin solvent adhering to a surface of the sintered compact, thereby exposing a matrix as sintered on a surface of the sintered compact. Pores existing in portions other than the surface are sealed with the resin; and dynamic pressure grooves are formed on at least an inner peripheral surface or an end surface of the sintered compact. [0013] The present invention further provides a fluid dynamic pressure bearing device having a thrust sintered bearing member, the sintered bearing member being produced by steps comprising: compacting a metal powder into a ring-shaped green compact; sintering the green compact into a sintered compact; impregnating a water soluble resin solvent into the sintered compact; and removing the resin solvent adhering to a surface of the sintered compact, thereby exposing a matrix as sintered on a surface of the sintered compact. Pores existing in portions other than the surface are sealed with the resin; and dynamic pressure grooves are formed on an end surface of the sintered compact. [0014] The present invention further provides a spindle motor comprising the above fluid dynamic pressure bearing device having the radial sintered bearing member or the thrust sintered bearing member, whereby the shape of the dynamic pressure grooves can be precisely transferred to a sliding surface, formation of scratches can be avoided when a pin is extracted in the production of a sintered bearing having three circular arc grooves, and leakage of the dynamic pressure can be avoided. [0015] In the production method of the present invention, for example, a sintered material having pores is dipped in water soluble resin solvent such as a resin solvent containing methacrylate ester monomer as a main component. Thereby the resin solvent is impregnated into pores by such a treatment. The resin solvent is more easily impregnated into the pores and seals them if an inside of a container in which the resin solvent and the sintered material are contained is evacuated, whereby air in the pores of the sintered material is removed, and then returned to atmospheric pressure. That is, the pores of the sintered material change from a low-pressure condition to an atmospheric pressure condition, and the resin solvent is impregnated into the pores. When returning to the atmospheric pressure condition, the resin solvent adheres to the entire surface of the sintered compact. Therefore, water is poured or sprayed on the sintered compact, and the surface of the sintered compact is washed and the resin solvent adhering to the surface is removed. [0016] In order to improve the shape and dimensional accuracy of the dynamic pressure grooves in producing a sintered bearing and to prevent leakage of the dynamic pressure, the resin solvent in the area from the surface of the sintered compact to a depth of approximately 0.1 to 0.2 mm is preferably removed by the washing. The resin solvent adhering to the surface of the sintered compact can be removed from the surface to an arbitrary depth by controlling the time for providing water. The sintered compact may be heated to a temperature of 80 to 90.degree. C., thereby hardening the resin in the pores after removing the resin solvent adhering to the surface of the sintered compact. [0017] In the production method of the invention, since the resin solvent which existed in the vicinity of the surface is removed and the working surface is a porous sintered material surface, the shape of the dynamic pressure grooves can be precisely transferred to the surface of the sintered compact. Furthermore, since the working surface is porous, the resin does not adhere to forming tools such as pins, scratches are not formed on the inner peripheral surface of the sintered compact when the pin is extracted from the sintered compact and the dimension of the sintered bearing member obtained after forming the dynamic pressure grooves can be precise. The resin solvent inside of the sintered compact is remained and hardened, thereby the pores of the sintered compact are sealed and leakage of dynamic pressure can be avoided. [0018] Thus, in the fluid dynamic pressure bearing device having the sintered bearing member obtained by the production method of the invention can support high loads, whereby a shaft can smoothly rotate with high precision. Therefore, when the fluid dynamic pressure bearing device of the invention is used in spindle motors used for magnetic disk devices, optical disk devices, and laser printers, and the like, precision rotation is obtained, whereby high packing density of information devices and high speed and high accuracy of digital copy machines can be obtained. BRIEF DESCRIPTION OF DRAWINGS [0019] FIGS. 1A and 1B are side cross-sectional views showing different examples of a spindle motor used for a magnetic disk device. [0020] FIG. 2 shows a production process for a sintered bearing member 32 and 42 shown in FIGS. 1A and 1B. [0021] FIG. 3 is a plan view showing an example of a shape of dynamic pressure grooves of a sintered bearing member. Continue reading about Production method for sintered bearing member, fluid dynamic pressure bearing device, and spindle motor... 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